1. Field of the Invention
This invention relates to a process for producing a non-combustible gypsum board and a non-combustible laminated gypsum board.
More particularly, it relates to a process for producing a high quality, non-combustible gypsum board and non-combustible laminated gypsum board, by molding, setting and drying a mixture of a powdery gypsum consisting of 50 to 95 parts by weight of a hydraulic gypsum and 5 to 50 parts by weight of gypsum dihydrate (100 parts by weight in total), with definite amounts of fibers and a setting retarder and a necessary amount of water.
Further, this invention relates to a non-combustible laminated gypsum board and a process for producing the same. More particularly it relates to the above board obtained by subjecting a mixture of a powdery gypsum, fibrous materials, a gypsum setting retarder and water to sheet-making and subjecting the resulting green sheet to lamination and press molding before a hydrating gypsum in the green sheet is hydrated.
2. Description of the Prior Art
Gypsum boards have been widely used particularly as interior materials since they can be economically utilized and are superior in fire resistance, sound insulating properties, heat insulating properties, dimensional stability and processability among inorganic, noncombustible building materials. The basis of gypsum board production techniques consists in adding water to calcined gypsum obtained by dehydrating gypsum dihydrate on heating, followed by molding and setting the mixture to thereby obtain a sheet-form material of a definite shape. Thus, gypsum dihydrate having no set properties cannot be used as gypsum in principle, and if gypsum dihydrate is present in admixture with calcined gypsum even in a few %, the strength of the resulting molded products is not only notably reduced, but also in the case of paper-covered gypsum boards, adhesion between paper and gypsum core is lost. Besides, since gypsum dihydrate promotes gypsum hydration, the amount of the setting retarder blended is increased for relieving the function; thus such use of calcined gypsum with gypsum dihydrate has been neglected (see Japanese patent publication No. Sho 57-49004/1982). Accordingly, if it is possible to produce an equivalent gypsum board to known ones from a powdery gypsum obtained by replacing a considerably large proportion of calcined gypsum by gypsum dihydrate, it will be possible to save an enormous heat energy required for producing calcined gypsum from gypsum dihydrate.
On the other hand, there have been proposed certain gypsum boards obtained by using no calcined gypsum but using gypsum dihydrate as it is and molding and setting it and processes for producing the same. However, any of these could not have been practically used as building materials, since, for example, .circle.1 the bending strength of gypsum boards obtained are only about 10.about.35 Kg/cm.sup.2 in spite of needing applied pressures as extremely high as 300 Kg/cm.sup.2 or greater (see Japanese patent publication No. Sho 49-31012/1974) or .circle.2 although press molding is possible under pressures as relatively low as 10.about.250 Kg/cm.sup.2, the bending strength of the resulting products are at most as very low as about 5.about.30 Kg (see Japanese patent publication No. Sho 55-349/1980).
In order to overcome these difficulties of well-known gypsum boards, there has been proposed a process for producing fibers-incorporated gypsum boards from a slurry having a calcined gypsum and fibers mixed with and dispersed in water, according to sheet-making process (see Japanese patent publication No. Sho 57-49004/1982).
The present inventors have made extensive research in order to solve the above problems of the prior art, that is, in order to achieve energy-saving in the aspect of raw material by utilizing gypsum dihydrate in the production of gypsum boards, in order to relieve the production conditions of products wherein gypsum dihydrate is used therein and in order to improve the strengths of such products. As a result, we have found that when gypsum dihydrate is used in a proportion of 5.about.50 parts by weight based on a powdery gypsum used in the fibers-incorporated gypsum boards, and hydraulic gypsum is used in a proportion of the remainder, that is, 95.about.50 parts by weight, then surprisingly enough, the resulting gypsum boards are not inferior in the practical strengths to those obtained by using hydraulic gypsum in 100%, and the production process can be carried out smoothly and efficiently; and based on this finding, the present invention has been completed.
Further the present inventors have made extensive research on the technical problem of the above-mentioned fiber gypsum boards. As a result, we have found that when a green sheet of 6 mm thick is prepared employing a conventional sheet-making apparatus, two or more of the green sheet are laminated before the hydraulic gypsum in the green sheet is hydrated, press-molded under a pressure of 30.about.500 Kg/cm.sup.2, and set and dried, then the resulting laminate is integrated, the tensile strength between the layers and the laminated gypsum board is not different from that of the unlaminated thin sheet, a thick sheet having a desired thickness can be obtained and also the production can be carried out at a similar sheet-making velocity to that in the case of thin sheet production and alike efficiently; thus the present invention has been completed.
As is apparent from the foregoing, an object of the present invention is to provide a novel process for producing a gypsum board, and more particularly it is to provide a process which can at the same time use hydraulic gypsum and gypsum dihydrate as raw material gypsum powder, can be carried out smoothly and efficiently and can yield a product having practical strengths.
Further, as is apparent from the foregoing, another object of the present invention directed to a fiber gypsum board in another aspect is to provide a thick fiber gypsum board (non-combustible laminated gypsum board) having fire resistance, sound insulating properties, heat insulating properties, high impact properties and other practical physical properties which could not have been obtained with conventional fiber gypsum boards (thin board), and a process for producing the same efficiently. Still another object of the present invention is to extend concrete application fields of fiber gypsum boards.